You can state that the bans have nothing to do with saving the reefs all you want, and it might even be true, probably is. That doesn’t mean it doesn’t help a little overall, because I bet poaching and unsustainable practices run rampant in most export nations (Oz likely excluded). You don’t actually know because you don’t have the required data to know, but you hope, I hope too, but hope isn’t enough. You want that to be true, so do I, but we don’t know if it’s true. Just like your statement that the reefs are too big to be impacted isn’t really empirical (or likely accurate, as a tiny impact in a vast system may not be particularly consequential, it’s still an impact, and this system just so happens to be shrinking at an absurd pace with no end forecasted until it’s gone). You can also compare the yellow tang situation to corals if you’d really like, it’s seductive, many would be swayed by it’s simplicity and emotional appeal, but without actual data (and really, probably even with it tbh), the comparison is empirically useless. As positive for the advocacy of the continued wild collection of that particular fish species as it may be, it really provides no support for the sustainability of collecting stony coral, it is apples and bowling balls...
The nihilist in me could justify the continued wild collection of coral. I mean, it’s all going to be dead in a geological time frame eye blink anyway thanks to us collectively. Continuing to collect is like adding another bulldozer to a fleet of a thousand, or putting another pin hole in blimp already mortally riddled with them. The existentialist in me would look for meaning in continued collection, perhaps since the reaper that we’ve so aptly conjured has already been set loose to flay the oceans, and since we as a species apparently won’t stop him and quite likely can’t anymore anyway, perhaps our collections could serve as the last bastions of the beautiful natural world that once was...